Question About Front Row Replacements

Ok then, when a prop or hooker is binned I fully understand that at the next scrum they have to bring a front rower on for the scrum normally at the expense of a backrower. However we have on some occasions then seen the team down to 14 bring a centre in at backrow to bring the scrum back up to 8 and the backs down to 6 which is a calculated risk.

However why don't teams just take a back off and leave the actual backrower on, is this allowed? Should the offending team be allowed to bulk the pack back up to 8 etc etc.

Hope that makes sense

In U19 rugby there is a law variation that the numbers in the scrum must be equal at all times even after a yellow card -

Law 20 : Scrum
20.1 Forming a scrum
(e)
In an 8 person scrum the formation must be 3-4-1, with the single player (normally the Number 8) shoving on the 2 locks. The locks must pack with their heads on either side of the hooker.

Exception: A team must have fewer than eight players in its scrum when the team cannot field eight suitably trained players in its scrum due to either the team not fielding a complete team, or a forward player being sent off or temporarily suspended for foul play, or a forward player leaving the field because of injury.

There is ni such requirement at senior level - 20.1(e)

Exception: When a team is reduced to fewer than fifteen for any reason, then the number of players of each team in the scrum may be similarly reduced. Where a permitted reduction is made by one team, there is no requirement for the other team to make a similar reduction. However, a team must not have fewer than five players in the scrum